In order to pump oil from a well in which casing has been set, the casing is perforated by a suitable gun at those levels in which oil has been indicated to exist. After the casing has been perforated a pump is lowered on the end of the conventional drill pipe string to this particular level. The pump includes a pumping tube a portion of which is sometimes referred to as a barrel within which a reciprocating plunger operates. This plunger is driven by a polish rod extending upwardly through the pumping tube to connect to the end of sucker rods which pass up through the drill pipe string to the surface of the well. Oil is pumped from the lower annulus adjacent the perforations in the casing through the pumping tube and up through the well pipe string to the surface of the well.
After pumping has been completed, or if it is desired to change the pump or repair it, it is necessary to pull the pipe string to bring the pump to the surface. Since, however, the pumping tube as well as the pipe string is filled with oil throughout the length of the pipe string, whereas the surrounding fluid in the well annulus defined between the exterior of the pumping tube and pipe string and interior of the casing is at a relatively low level, there exists a very large pressure differential rendering it extremely difficult to pull the pipe.
The foregoing condition can be overcome if the fluid or oil within the pipe string and pumping tube could be passed in a reverse direction to the annulus exterior of the pumping tube and string. If the fluid could be passed in this manner, the large hydrostatic head created within the pipe string could be "dumped" into the surrounding annulus of the well and thus equalize the pressures involved so that removal of the pipe string without the large hydrostatic head of oil could be readily achieved.
My U.S. Pat. Nos. 3,168,873 issued Feb. 9, 1965 and 4,047,853 issued Sept. 13, 1977 disclose hydraulic unloading and circulating devices which solve the foregoing problem. However, the solutions described in these patents are not suitable for certain types of oil well pumps.
More particularly, there has been introduced an improved type of well pump capable of delivering a greater capacity of oil through a pipe string than previously available pumps. One such improved type pump is provided by the Heavy Oil Producers Service, Inc. and are referred to in the art as HOPS type pumps. Pumps of this type include a relatively large diameter pumping tube which connects to the extreme end of the drill pipe string rather than being received within the drill pipe string. For such a pump, it would be desirable to incorporate some type of hydraulic unloading or pressure equalizing means in the pumping tube itself. Further, it would be a desirable feature to be able to operate the "dumping" or equalizing device from the surface of the bore hole and so design the device that it will provide continuous communication between the interior of the pipe string and well annulus while pulling the pipe.
A further consideration with respect to the foregoing would be provision of such a hydrostatic equalizing device which could be incorporated in the newer type referred to pumps with a minimum of structural modification all to the end that overall economy is realized in the manufacture and use of the device.